Footprints of underground mines are critical for many geotechnical and environmental projects. Geophysical detection and mapping of mine workings depends on the physical property contrast between the mine and country rock, and the mine depth and dimensions. Air-filled voids provide a density and seismic velocity contrast with the surrounding rock. Water-filled voids represent a density, electrical resistivity, and seismic shear-wave velocity contrast with rock. Collapsed mine voids might be detectable with microgravity, electrical resistivity, seismic refraction, multi-spectral analysis of surface waves, electromagnetics, ground penetrating radar, or seismic reflection. Cross-borehole geophysics can detect mines at great depths with properly located boreholes. Careful technique selection is essential to mine mapping success.
Myth or Reality: Old Mine Voids Beneath a New Building? Survey Says…
One of our clients planned to develop a vacant multi-acre property into a manufacturing facility, but background research indicated that portions of the site had been mined for coal decades before. There was no indication from the ground surface – it was an open field – but the possible presence of underground mines could change the entire site design.
To determine whether mines could exist underground, RETTEW performed a combined microgravity and electrical resistivity imaging (ERI) survey in a quasi-tic-tac-toe pattern across the site, focusing on the planned building footprint (Figure 1). Microgravity is sensitive to differences in subsurface density, and mined areas could appear as mass-deficient areas relative to the surrounding intact, unmined rock. ERI measures electrical contrasts; air-filled mines could appear as electrically resistive anomalies and water-filled mines could appear as electrically conductive anomalies relative to the intact bedrock.
The microgravity plan-view contour map (Figure 2) showed a distinct mass-deficient zone (red shading) in the eastern half of the site, and the ERI cross-sectional profiles (Figure 3) over the same portion indicated both electrically resistive and conductive anomalous zones. Data from the microgravity and ERI surveys were superimposed, and the results depicted specific areas of concern – possible air- or water-filled mine voids.
This multi-technique geophysical survey pinpointed areas for follow-up testing with geotechnical borings to determine sources of the identified anomalies rather than simply putting borings in gridded or random locations across the site. The survey results helped our client use their site investigation budget effectively so areas of concern could be remediated or the design altered to avoid them.
Meet Chris Brixius
Geotechnical and Geophysics Team Lead
Discover how Chris Brixius, RETTEW’s Geotechnical and Geophysics Team Lead, combines nearly three decades of engineering expertise with a passion for adventure, mentoring, and family. Go “beneath the surface” and get to know Chris!